<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <script src="../js/vue.js"></script>
</head>

<body>
  <!-- 
    监视属性watch：
        1.当被监视的属性变化时，回调函数自动调用，进行相关操作
        2.监视的属性必须存在，才能进行监视!!
        3.监视的两种写法：
            (1) .new Vue时传入watch配置
            (2) .通过vm.$watch监视
   -->
  <div id="root">
    <h2>今天天气很{{weather}}</h2>
    <!-- <button @click='isHot = !itHot'>切换天气</button> -->
    <button @click='changeWeather'>切换天气</button>
  </div>

  <script>
    Vue.config.productionTip = false

    const vm = new Vue({
      el: '#root',
      data: {
        isHot: true
      },
      methods: {
        changeWeather() {
          this.isHot = !this.isHot
        }
      },
      computed: {
        weather() {
          return this.isHot ? '炎热' : '凉爽'
        }
      },
      watch: {
        weather: {
          immediate: true, // true：初始化时让handler调用一下
          handler(newValue, oldValue) {
            console.log('weather被修改了', oldValue, '改成了', newValue)
          }
        }
      }
    })
  </script>
</body>

</html>